Treated timber often contains toxic metals such as arsenic, chromium and copper. Use less harmful alternatives as extensively as possible when you are building something in your garden, and you will protect both yourself and the environment . Pressure impregnated timber has been treated with biocides to prevent decay. The real environmental problem arises when the pressure impregnated timber becomes waste. If the timber goes astray, the toxic substances are spread and can cause environmental damage. Since arsenic and chromium cannot be destroyed by combustion, the timber must be disposed of in special dumps or in plants where it is burnt with high-efficiency flue gas cleaning.